In a message dated 6/14/08 2:00:43 P.M. Eastern Daylight Time,
bartb@xxxxxxxxxxxxxxxx writes:
I have 8 flying electrodes on my disc. Each of these flying
electrodes is a 3/8" x 1.5" long solid tungsten rod.
Some notes with bare tungsten press-fit to G-10:
When I assembled the 3/8" tungsten rods into my 12" G10 disk (sound
familiar?) I did some testing in a scrap piece of G10 (the corners
of the squares
I cut off to make a circle from the square piece of 3/4" thick G10). I
drilled a hole with the same distance-to-edge as the actual rotor
holes and reamed
it out to .373" to begin with. The rods were .375" dead-nuts, to the
best of
my measuring ability. I had a .374" reamer in case they ended up too
tight.
It took about two tons of force to get the rods into the holes, or
to adjust
them once they were in. So I reamed all 8 rotor holes to .373".
Another thing I wanted to check was the retaining ability of G10 at
elevated temperatures. So with a torch and a non-contact thermometer
I carefully
warmed a rod up to 300 deg F (past the 284 deg F max service temp of
G10!).
Guess what? That press-fit rod now freely slid back and forth in its
hole!
After it had cooled, it was nowhere near a tight fit as it had been.
There has been discussion here that adding a retaining setscrew
thru the
rotor edge actually weakens the G10. Maybe the best bet is clamping
shaft
collars:
From McMaster-Carr (_www.mcmaster.com_ (http://www.mcmaster.com) ):
6157K13
One-Piece Aluminum Clamp-on Shaft Collar 3/8" Bore, 7/8" Outside
Diameter,
3/8" Width
In stock at $2.25 Each
or if you prefer thermal mass over conductivity:
6435K13
One-Piece Clamp-on Shaft Collar Black-Oxide Steel, 3/8" Bore, 7/8"
OD, 3/8"
Width
In stock at $1.99 Each
The plain steel would probably suffer in that environment. They also
offer stainless steel for substantially more cost and lower thermal
conductivity.
I've seen some folks use shaft collars as the sole means of
retention on
their RSG's. Of course you'd have to re-balance after adding the
collars to
both sides of the rods. Would add about $40 for my 8-electrode RSG, but
certainly seems worth it if it keeps the tungsten slugs from
escaping! Might have
to watch out that the collars don't arc to the stationary electrodes.
Some folks use rings on the faces of the disk, but I dunno how they
secure the rods. I also dunno how you would get heatsinking from it
unless you
tack-welded the collars to the disk after everything was assembled.
Maybe the best would be to use a solid aluminum rotor with an
insulated
hub or belt drive. I've got all the parts for a timing belt drive,
but I
figured I'd start simple.
-Phil LaBudde
Center for the Advanced Study of Ballistic Improbabilities
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